I posted a reply to jpaana but it didn't get through :

I followed very carefully your procedure without success.
I thought that maybe a careful polar alignment of the scope is a requisite to a successful slaving.
My polar alignments have always been sloppy and I'm waiting clear skies to do a better one.
This idea is reinforced by the remark of fcasarra : "May be your telescope is just a bit of north(?)"

fcasarra said "sign of the OTA parameter (+ is west, - is east)"
As I understand it the OTA offset is defined as the distance between the intersection of the RA and DEC axis and the scope optical center : it should not have a sign + or -
I am wrong ?

Well, sort of.
The OTA offset and the pier side are connected and if the pier side is incorrect this can be compensated for by changing the sign of the OTA offset. One way this could happen is in the Southern hemisphere where the dome is upside down.

Here's a process that might help:

1. Make sure that the dome azimuth movement and position are correct and repeatable. If the azimuth is incorrect or not repeatable you will never get a good sync. Good means to withing a couple of degrees.
2. Align the mount. For a celestron GEM that's polar aligned a quick align should be good enough but once again you can't get a good sync without a good alignment. Within a degree should do.
3. Set the dome radius to a sensible value, Do not change it again.
4. Set the mount to hour angle 6h, declination 90. This is the Align position, looking at the pole with the counterweight shaft down. In this position the OTA offset will have no effect on the dome azimuth, not will the NS position of the mount.
5. Adjust the mount EW offset until the scope is looking through the centre of the dome slit.
6. Move the mount so the counterweight shaft is horizontal and the OTA is looking at the Southern horizon. In this position the main things that affect where the dome needs to be are the EW mount offset and the OTA offset. We have already set the EW offse so set the OTA offset. Try this with the mount on both side of the meridian and iterate as required to get the scope able to see out in both cases.
7. set the scope so it is looking at the East or west horizon, this should be with the dec counterweight shaft pointing down. Adjust the Ns offset so the OTA can see out. Do this both looking East and West.

That should get the EW offset, the OTA offset and NS offset set and each one is set independently of the others.
I'm not sure how to manage the up/down offset but maybe adjusting things with the mount pointing at an altitude of about 60 degrees would do this.

The important thing is to only adjust a parameter when you are in a position where it has a big effect.

Hope this helps.

Hi Ferran,
Thanks for sharing the spreadsheet .
I'll play with it to-morrow and I'll keep you informed

OTA sign is for the SpreadSheet only!
In INDI, OTA is always positive.

Last night was dark enough to improve the polar alignment of my scope (I was really out !).

I’m not very good with this kind of exercise but autoguiding does miracles.

Once the polar alignment was better, I followed the process provided by Chris ROWLAND above to set the dome parameters … and the dome synced perfectly with the scope in all directions !

I improved also the park azimuth of the dome which was some degrees off.

Thanks to jpaana, Ferran and Chris for their help.

I'm having a similar problem with my NexDome - but one that is giving me a headache because it's only a problem with the INDI dome driver, the dome driver that comes with TheSkyX that I am using my trial license works perfectly using exactly the same dimensions.

I will try at @chris-rowland suggestions to make sure I'm not misunderstood how INDI's dome module differs from TheSkyX. There's only one other thing that I think that could be confusing things and that is that the Paramount driver might be reporting detail that is causing the problem eg: due to pier side calculations.

I'm Southern Hemisphere just in case that means I need to sue different witchcraft - the documentation around this stuff isn't exactly clear or that easy to find!

I used a few spare minutes during teh daylight hours today to do some testing using the dome simulator vs. the NexDome driver as installed on TSX and here's an example of one of the pointing "errors" - I'm wondering if there's enough diagnostic information to figure out why.

Note I'm at -37'

Offset settings are:
- Radius - 1.0
- E/W - 0.0
- N/S - 0.1 (slightly north)
Up/Down (Z) - -0.12

These I take to be relative to the pivot points for the intersection of the RA/Dec axis with the OTA Offset measuring how far away from that pivot point the optical centre of the OTA is.

Attached is a screen shot of one example of a significant difference.

The picture tells me nothing because it isn't clear what the dome position is or what you expect it to be.

Your offsets do not mention the OTA offset, just the position of the mount reference relative to the dome centre. If you are using a GEM and the OTA offset is not set correctly then your dome position will be incorrect.

The mount reference point is as you suggest the intersection of the hour angle and declination axes.
The OTA offset is the distance along the declination axis from the mount reference to the centre of the telescope, This, with the other mount offsets and the current Ra and Dec defines the line along which the telescope needs to be looking.

Here is a list of tests of dome synchronisation that may help in diagnosing the problem. In all cases what is needed is the mount OTA position and look direction, where the dome azimuth actually is and where you would expect the dome azimuth to be.

0. Check that the dome azimuth positions are correct and reproducible. Azm 0 should be N, 90 should be E, 180: S and 270: W.

1. Mount at Ha 6 dec 90 (-90 in the S). The OTA should be above the mount, looking at the pole, with the counterweight shaft straight down. In this position the OTA offset should not matter and the main control over the dome position is the EW offset.

2. Mount at Ha 0, Dec so it is looking at the horizon. In this position the OTA is to one side of the mount, with the counterweight shaft horizontal. The pier side will dictate if the OTA is East or West of the mount. In this position the main control is the OTA offset.

3. Mount with an Ha of +6h or -6h and Dec 0. In this position the OTA is looking East or West with the counterweight shaft straight down. The main control is the NS offset.

I suggested setting the OTA offset to a negative value. This is because it could compensate for errors in the dome position caused by the pier side being incorrect or problems with the algorithm in the southern hemisphere. If it can't be set negative then it would be worth allowing this in the dome settings.

Hope this helps. It needs a methodical approach where positions are chosen that isolate a single mount parameter, both for set up and diagnosis. Anecdotes and pictures at random positions don't help much.

Hi,

You can replicate the Dome syncing calculus in this google spreadsheet:
Dome Syncing calculus

(Usage: You can enter the values in blue cells, and output are in the orange cells.
You enter your telescope parameters, and your latitude. Also enter target declination and target hour angle (angular distance to the meridian, in hours).
Notes:
* In the spreadsheet, OTA must be set positive if telescope is in the west side, and negative if it is in the east side. In the driver OTA is always positive since the change in sign is set done by the pier side button.
* The units in the blue cells are in cm, but in the driver are in meters.

I already put your parameters (except I guessed OTA distance, and latitud is not exact) and the results are the following:
With the telescope on the West side, Target Azimuth = 288
With the telescope on the East side, Target Azimuth = 332

So, the pier side seems the most probably cause.

Check if you set the pier side right.
If you have done right and the result still is wrong, let me know. Maybe there are one issue when the telescope is in the suthern hemisphere.

Regards,
Ferran Casarramona

Thanks Chris - apologies, curse of knowledge - I knew what was salient and assumed everyone else could read my mind!

I did read your methodical approach earlier, it's a great idea and with your permission will add this as a section to either the Dome driver page or to to the manuals/tutorial pages, it's a death trap for youngsters like me.

I'll try your proposed routine tomorrow when the weather clears, some of course I have checked prior to posting.
0. - done
1 - done
2 - 3 - homework for when the rain stops (there's no way I'm going out in THAT!)
"4" - on a hunch I did try going with a negative OTA Offset, it worked in some directions but not others - head scratcher to be sure.

The point of the image is/was that TheSkyX seems to nail the dome slot location perfectly every time but INDI's has been variously super close or wide off the mark despite getting out my laser disto, spirit levels and in the end a good margin of sodding about changing values - all without any degree of causality I could detect.

Plug exactly the same (as measured) distances into TheSkyX dome driver and it nails it perfectly every time.

Looking at Ferran's reply above, I suspect his conclusion is correct and is ANOTHER problem with using a Paramount, from what I have read on the MaximDL forums, it's not great about reporting what pier side it is on (to be totally frank, I have still to work out what "pier side" actually means, the OTA moves from one side to the other depending on where it's pointing! - Reading ASCOM's definition helped me not at all as there are 5 other "experts" who have different views on the same topic :-S )

Which leads me to conclude that maybe I don't have my mount setup correctly (seems unlikely given that my PA is bang on) or that I really have no clue what I am doing and should condor taking up darts.

Thanks Ferran - I had started to play around with the calculator and got a different result again, I realised it was because I misunderstood what you meant by "angle" in the field - HA, seems obvious now you mention it. Thanks so much for sharing it - amazing!

You confirmed my hunch, I noticed that when I was doing some early testing before my rotator kit arrived the answers coming out of the simulator were perfect, but I was using my EQ8-R back then. Since adding the Paramount I started getting crazy results- making me think it wasn't something obvious.

My guess is that the Dome class/driver "sniffs" the Mount class/driver and figures out teh pier side from that. Is that a good assumption?

From what I have read in the Diffraction Limited forums for Maxim DL, the Paramount/TheSkyX system doesn't seem to play nicely like that - which might explain why their native driver is fine and INDI is struggling - but until I go through Chris' diagnostics I won't be sure.

This is all assuming I understand the concept of pier side at all - I have to admit every time I think I do, I read something that confuses me more.